This invention relates to resistor elements of a layered structure which may be used as a chip-type thermistor or a chip-type resistor element. More particularly, this invention relates to such resistor elements having mutually oppositely facing pairs of inner electrodes inside a resistor base body. This invention relates also to methods of producing such resistor elements.
It has been known to use chip-type thermistor elements as a temperature-sensitive element or an element for temperature compensation. Elements of this type having different resistance values are frequently required, depending on where they are used. In response to such a requirement, chip-type thermistor elements of different structures have been proposed. Japanese Utility Model Publication Jikkai 6-34201 and Japanese Patent Publication Tokkai 4-130702 have disclosed various kinds of chip-type thermistor elements using a sintered ceramic body obtained by sintering together a ceramic material with inner electrodes.
FIGS. 10 and 11 show, as an illustration, the structure of a prior art thermistor element 151 of such a layered structure having a sintered ceramic base body 152 comprising a semiconductor ceramic material with a negative temperature coefficient. Mutually opposite end surfaces of this sintered ceramic body are referred to, for convenience, as the first end surface 152a and the second end surface 152b. Outer electrodes 159 and 160 are formed so as to cover the first and second end surfaces 152a and 153b, respectively. A set of horizontally extending inner electrodes (referred to as the first electrodes) 153, 154 and 155 are formed at different heights inside the sintered ceramic body 152 so as to be exposed to the exterior on the first end surface 152a. Correspondingly, another set of horizontally extending inner electrodes (referred to as the second electrodes) 156, 157 and 158 are formed respectively at the heights of the first electrodes 153, 154 and 155 inside the sintered ceramic body 152 so as to be exposed to the exterior on the second end surface 152b, the electrodes 153 and 156 forming a pair, the electrodes 154 and 157 forming another pair, and the electrodes 155 and 158 forming still another pair. Each pair of first and second electrode is in a coplanar relationship and separated by a gap of a same specified width and is designed such that the gaps between these three pairs of inner electrodes overlap in the vertical direction, that is, the direction of the thickness of the sintered ceramic body 152.
The resistance of the thermistor element 151 thus structured is adjustable to a desired value by varying the size of the gap between the aforementioned first and second inner electrodes as well as the number of pairs of first and second inner electrodes. In order to accurately set the resistance value of the thermistor element 151, therefore, it is necessary not only to highly accurately set the gap between the first and second inner electrodes of each pair but also to form each inner electrode 153-158 such that the gaps therebetween are all accurately positioned in the direction of the thickness of the sintered ceramic body 152. In other words, strict process management was indispensable for the production of chip-type thermistor elements having a desired resistance value.
When chip-type thermistor elements having different resistance values are desired, either the gap between the first inner electrodes 153-155 and the second inner electrodes 156-158 or the number of layered pairs of inner electrodes must be changed. If the width of the gaps is to be changed, however, a different electrode pattern must be prepared and printed on ceramic green sheets with a conductive paste in order to obtain sintered ceramic bodies by the conventional integral sintering technology. Since the accuracy involved in the printing of conductive paste cannot be improved beyond a certain limit, variations in the resistance values of the thermistor elements thus obtained are significantly large, and the center of distribution of these resistance values tends to be significantly far away from the desired value. In other words, the yield of acceptable products is not sufficiently high, if it is desired to produce resistor elements with resistance values having only small variations.
Because the gap size and the accuracy in overlapping layers must be strictly controlled if a desired resistance value is to be accurately attained, as explained above, it becomes very expensive to produce chip-type thermistors with many different resistance values. Problems of this kind have been in existence not only with thermistor elements but also with varistors and fixed resistors with a similar inner electrode structure.